Frequency response circuits



Feb. l5, 1949. R. w. BEcKwlTH 2,461,956

FREQUENCY RESPONSE CIRCUITS Filed oct'. 10 1946 Fig. l.

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Inventor: Folcnert4 W. Beckwith,

v bym @W- His Attorney Patented Feb. 15, 1949 STATES ATENT orner:v

FREQUENCY RESPONSE CIRCUITS v RobertW. Beckwith, Syracuse, N. Y., assignorto General Electric Company, za corporation of New York ApplicatonOctober 10, 1946, SerialNo. 702,377

4 Claims. (Cl. -Z50-"27) band systems heretofore employed, difficulty has been experienced dueto the Aeffect of interfering noise whichnormally has substantial level in carrier-current systems. Furthermore, in certain power systems, such great use is made of carriercurrent over power lines'thatfrequency'congestion is a serious problem, and the need is evident for a narrow 'band frequency-shift system.

I have found in actual practice that a much narrower band width and frequency spacing can 'be satisfactorily employed for remote rtelemeterin'g over'power lines, with an improvement' in signalto-noise ratio of as high as 25 db. andan increase by as much as twenty times in the number lof 'individual Vchannels that maybe placed inia jgiven frequency spectrum, if Vthe frequency selection circuits are 'given'the proper characteristics in accordance with my invention. Specifically, I employ a capacitive bridge network utilizing a piezo-electric element for discriminating between .high-frequency signals whose 'frequencies are relatively' close together.

Itis accordinglyan objectof my invention to rprovide improved frequency response :circuits 'which are particularly adapted for 'use ina frequency shift system for carrier-current telemetering over power lines.

It is another object of my invention to provide an improved frequency discriminator which vis sharply selective, thereby permitting closer frequency spacings between adjacent channels and also providing substantial improvement 'in' reliability Vunder adversenoise conditions or interfering signals.

It is also an object of my invention to provide improved frequency selection circuits, particularly adapted for use in frequency shift transmission system, which give a higher signal-'to-noise ratio than heretofore and permit a substantial increase in thenumber of frequencychannels available.

vIt is still further an object of my invention to provide improved crystal discriminator circuits. The features of myinventionwhich I believe to I'be novel are set forth Vwith particularity -in the appended-claims. My invention itself, however, together with further'objects and advantages thereof, may best be understood Ybyreference to rthe following description taken in connection with the accompanying drawing in which Figs. 1 and 2 are simplified block diagrams of frequency-shift carrier-current transmitting and receivingapparatus, respectively, which illustrateone particularly advantageous use for the circuits ofmy invention; Fig. 3 is a circuit Adiagram of "a crystal "iilter particularly adapted for use in the ^system-of Fig. 2; Fig. 4 isa graphicalrepresentation'illustrating the frequency characteristics of the circuitf'of Fig. 3; Fig. 5 is Aa circuit diagram of a crystal discriminator embodying the'principles of my invention andwhich may also be particularly usefulin the 'system of Fig. 2; and Figp is a graphicalA representation illustratingthefrequency characteristics of the circuits of Fig. 5. The frequencyg'shift transmitting system shown liniFigQ--l is illustrated 'as comprising twocrystalcontrolledoscillators I0 and l Ifor generatingfrequencies f1 yand 'f2 used for transmission. These frequencies are-alternately supplied through the buffer amplifiers I'2 and i3 respectively to the final amplifier lll, the output of which is supplied to 'the power lines I5 through the usual carrier- `current tuning and coupling elements I6.

The shift back and forth between frequencies f1 and f2 is accomplished at a coded rate yin ac- 'cordanceV with intelligence to be transmitted,

as is well'understood inthe art. In Fig. 1 a switch l1 is conventionally represented as being'actuated bycontroliing apparatus (not'shown) alternately to energize amplifiers `l2 and |3. Of course velectronic switching or any Vother equivalent means for alternately transmitting frequencies f1 and 'f2 lat the coded rate may be employed.

vIt is Vcommon practice to transmit a'plurality of coded signals Ysimultaneously over the same pair of power lines from a plurality of transmitters, each of 'which may `be similar to `thatillustrated bythe-'block diagram'of Fig. 1. These other transmitters will of course operate at different pairs vof vfrequencies within spaced frequency channels, and their outputs are'merely indicated -conventionally as'being supplied to thepower'line l5 over other tuning and coupling elements Ia, lb, etc. The same `coupling elements 16 rmay optionallybe utilized for a'plurality of channels, because of the close spacing that vcan'be used in jaccordance with myinvention.

In the carrier-current `receiving apparatus illustrated in `block form in Fig.y 2,' thepairs -of :fre-

quencies representative of the codes being transmitted are supplied to the receivers through the tuning and coupling elements 20, a, 20h, etc. As in the case of the transmitters, each may optionally be used for a plurality of channels. For reasons that will become more fully apparent later, the frequencies f1 and f2 used for each channel are first selected by a crystal filter 2| having a very sharp selectivity characteristic, as indicated by small curve 22, also shown in greater detail in Fig. 4. This filter passes only a relatively narrow band of frequencies including f1 and f2 -and excludes frequencies outside this band to prevent false operation due to noise or unwanted signals from other channels. The selected signals `may then be further amplified in amplifiers 23 and 24 and any amplitude modulation present in the signals is then removed in the limiter amplifier 25. These ampliers may be conventionally coupled together through double-tuned transformers 26 and 21 which will normally have a much broader frequency characteristic than the crystal filter 2l, as is indicated by the small curves 28 and 29.

In order to reproduce the transmitted code signal, a very sharply selective, crystal discriminator 30, which will be described in greater detail below, is employed having av frequency characteristic indicated by the small curve 3| in Fig. 2, also shown in greater detail in Fig. 6.

As shown, whenever, the frequency f1 is received, a positive potential is developed at the output of the discriminator 30, and whenever the frequency ,f2 is received a negative voltage is developed. These unidirectional control potentials may then be utilized in any manner known to the art to effect the control or telemetering operation. They are indicated conventionally in Fig. 2 as being supplied to a relay control unit 32 which actuates a relay 33 to energize or deenergize controlled apparatus (not shown).

The arrangement and operation of the crystal filter 2| will be better understood by reference to Figs. 3 and 4 of the drawing. The filter is in the form of a bridge network with the high frequency signals fi-and fz supplied through transformer 40 and resistor 4| to a pair of diagonally opposite input terminals 42 and 43'. The output is taken across the opposite diagonal of the bridge,

between terminals 44 and 45, one of which may be grounded. The high-frequency currents may flow through either of two parallel paths between the input terminals 42 and 43, as indicated by the instantaneous current vectors i1 and i2. The current i1 flows through two bridge arms comprising a piezo-electric crystal 46 and a resistor 41 respectively, while the current i2 flows through the other two bridge arms comprising a variable capacitor 48 and resistor 4S respectively. It will be observed that the high-frequency potentials developed between the output terminals 44 and 45 are the result of the difference between the voltage drops across resistors 41 and 49 these being indicated by the vectors e1 and e2 respectively.

The crystal lter of Fig, 3 is adjusted to pass only a narrow frequency band including fi and f2. As indicated in Fig. 4 the selectivity curve of the crystal filter circuit is extremely sharp. In one frequency shift, carrier-current telemetering system embodying my invention, the frequencies f1 and ,f2 for one controlled channel were 130.00 and 130.105 kc. respectively, and the two frequencies employed for the nextA adjacent channel were 131.000 and 131.107 kc. respectively. The actual frequency characteristic of the filter used in the second channel is shown by the curve 22 in Fig. 4. To illustrate the sharpness of this curve, there has been superimposed upon it a theoretical curve I8 for a single-tuned circuit having the extremely high Q of 1300. It will be seen that the crystal filter gives the same shape of peak with even better sideband attenuation.

The operation of the crystal filter circuit 2| of Fig. 3 will be better understood when it is realized that the piezo-electric crystal 46 (including its holder) -appears as substantially a pure capacitance at frequencies removed from the narrow range in which it exhibits resonance characteristics. Therefore, it is possible to adjust the variable capacitor 43 so as substantially to balance the capacitive current through the crystal for frequencies above and below this range, so that the potentials e1 and e2 very nearly cancel each other. If the resistors 41 and 49 are equal, then of course the capacitance of capacitor 48 will be adjusted to be equal to the equivalent capacitance of crystal 4E with its holder. Under these conditions, no output will result at terminals 44 and 45 except in the region where crystal 46 goes through its well-known resonant characteristic. Since this characteristic is very sharply defined, the resulting selectivity curve for the filter is extremely sharp, as previously pointed out. Without the capacitor 48, a much broader selectivity characteristic would be obtained. Looking at it another way, the voltage e2 is practically constant over the operating frequency range and subtracts from e1 to bring the side of the selectivity curve towards zerovery quickly on either side of the resonance peak, as the bridge approaches a balance.

It will be appreciated by those skilled in the art that the output resistors 41 and 49 in Fig. 3 might be replaced by capacitors or inductances under some conditions. The input circuits through transformer 40 might also be series-resonated or parallel-resonated at the operating frequency to give additional sideband reduction, and in general it will be found that this is desirable since the input impedance can be matched to the source of signal and thus greater receiver sensitivity obtained.

The crystal discriminator 30 shown in Fig. 5, also comprises a four-arm bridge network somewhat similar to that of Fig. 3 but the frequency characteristics thereof are entirely different, as illustrated in Fig. 6. The input signals are supplied through the transformer 50 to the diagonally opposite pair of input terminals 52 and 53 while the output is taken across the terminals 54 and 55, one of which may be grounded as shown. As indicated by the current vectors I1v and I2, currents may flow from terminal 52 to terminal 53 through two parallel paths. The current I1 flows through the piezo-electric device 56 and a load impedance, indicated as a resistor 51, while the current I2 flows through the variable capacitor 58 and a load impedance, indicated as a resistor 59. Thecircuit differs from that of Fig. 3 in that unilateral conducting devices, represented as diode rectifiers 60 and 6|, are included in each of the parallel paths. These rectiers are poled to permit current flow in the same direction, i. e., from terminal 52 toward terminal 53. The resistors 51 and 59 are bypassed for the frequencies of the input signals by capacitors 62 and 63, as is conventional in diode detector circuits. Direct current return paths from the anodes of the detector 60 and 6| to ground are also provided by the resistors 64 and 65 in shunt to the crystal `56 and the variable capacitor 58 respectively,

This direct current return Vcould -be provided by inductances whose reactances are large compared tothe reactance of the capacitor 58.

It is well known in crystal theory that as the frequency applied to a crystal is raised from a frequency below the range in which it exhibits resonance phenomenay it first passes through a point `at uwhich Vit "looks 'lile a series-resonant circuit arifdwtlien'throughapoint at whichit looks like 'aparall'el resonant circuit, vwith the separation 'between these ts quitesmall. I take advantage of this phenomena, in accordan'ce'with my invention, by selecting the frequencies f1 and f2 close enough together so that the crystal 5% can be ground to exhibit these resonance characteristics in the vicinity of the two frequencies. As previously described in connection with the circuits of Fig. 3, the crystal looks like a substantially pure capacitance (considering the complete crystal assembly in its holder) for frequencies above and below this range.

With these facts in mind, it will be readily apparent how the crystal discriminator circuits of Fig. 5 are adjusted to produce the frequency characteristics shown in Fig. 6. The value of capacity 58 is adjusted to balance the currents I1 and uit I2 flowing through the two parallel paths for frequencies outside the resonant range of the crystal, just as in the case of Fig. 3. At the lower frequency f1, the current I1 through the crystal exceeds the current I2 through the capacitor 58 due to the lower impedance of the crystal. Therefore the resultant rectified potential E11 exceeds the potential E2, resulting in a positive potential at terminal 55 with respect to ground. The opposite is true at the higher frequency f2, in which case the potential E1 becomes much smaller than potential E2 due to the relatively high impedance of the crystal. Over the relatively narrow range of frequencies involved, the potential E2 is substantially constant.

It will also be appreciated that the frequencies f1 and f2 may be selected in some cases to lie closer together than those illustrated, in which case they would Afall at points closer to the horizontal axis of Fig. 6 along the sharply-sloped, center portion of the curve. However, I have found that greater frequency stability of the circuits, with greater freedom from the effects of temperature, humidity and other factors, is obtained by selecting these frequencies so as to coincide substantially with the positive and negative peaks of the curve, respectively.

It will be understood that the curves of Figs. 4 and 6 can be applied to other frequencies than those specifically shown if the fi-fz spacing remains approximately a constant percentage of their average frequency. Thus the horizontal scale may be expressed in terms of percent deviation from center frequency in a more generalized expression.

It will now be apparent that I provide simple and effective frequency selection circuits, particularly adapted to the requirements of a frequency-shift, telemetering system, in which the number of available frequency channels may be increased while at the same time providing greater freedom from noise interference and interference due to cross-talk between signaling channels. My circuits are also obviously adaptable to use in a space radio system, at the same or higher frequencies.

While I have shown particular embodiments of my invention and suggested certain modifications that may be made therein, it will, of course,

@fre-1a s foreeonterpiatefbytheappended @reinste-'cover vanyw'suclfi v"noticliilca'tionsjas fall within "the "true lspirit and-'scope offmy invention.

fjWhatlI, "claim as new"arid` desire to secure "by hett'ersv Patent of the U nitedStates is:

n 1. Infapparatus for discriminating between waves' of two different frequencieafthe combinatio'n of 4ap"'a:'l'r"of input 'terminals to vwhich said -ifi/"aves` are "supplied, a vrpair of `parallel current fp hsb'et'wensai'clterminals'each path including i a 'ifecti'f'ying "devicealrid a load impedance 'adapted to have unidirct nal 'potentials developed there- 'oninfresponse'to s d" waves,"eachpath als'oserialily 'including a 'curreiitcontrolling element, one element being a piezo-electric crystal exhibiting series resonance and parallel resonance near said two frequencies respectively and substantially pure capacity reactance aboveand below said frequencies, the other element being a capacity, output means responsive to the difference between potentials developed across said load impedances, said capacity being adjusted to impress minimum resultant output potential on said output means at frequencies for which said crystal exhibits substantially pure capacity reactance.

2. In apparatus for discriminating between waves of different frequencies lying within a relatively narrow frequency band, a pair of input ter- `minals for said waves, two parallel paths between said terminals, each path including a unilaterally conducting element and a load impedance adapted to have unidirectional potentials developed thereon in response to said waves, said elements being Apoled to permit current flow in the same direction between said terminals, one path further serially including a capacitance and the other path a piezo-electric crystal, said crystal exhibiting a relatively high impedance near one edge of said band and a relatively low impedance near the other edge of said band, means for adjusting said capacitance substantially to balance the currents through both paths at frequencies I outside said band, and output means responsive to the difference between unidirectional potentials developed across said load impedances.

3. Apparatus responsive to signals having frequencies lying within a predetermined band comprising, in combination, a four-arm bridge network having a pair of diagonally opposite input terminals to which said signals are supplied, first and second arms adjacent one of said terminals each comprising a resistance in parallel to a capacitance of low impedance at said frequencies, third and fourth arms adjacent said other terminal, said third arm serially comprising a capacity element and said fourth arm serially comprising a piezo-electric element, said piezo-electric element being series-resonant near one edge of said band and parallel-resonant near the other edge of said -ban-d, a pair of unilateral conducting devices also serially included in said third and fourth arms respectively and poled to permit current flow in the same direction between said terminals, means providing direct-current paths in shunt to each of said elements, means for a-djusting said capacity element substantially to balance the effective capacity of said piezo-electric element outside said band, and output means connected across said first and second arms.

4. In apparatus for discriminating between waves of different frequencies lying within a relatively narrow frequency loarid, a 'pair of input tertric device resonating within sai-d band and hav- 10 ing substantially pure capacitance outside said band, the other element ibeing a capacitance having a, value substantially to balance the currents in said two paths for said frequencies outside said band, and a utilization circuit responsive to the diierence in potentials on said load impedances.

ROBERT W. BECKWlTI-I.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,085,008 Crosby June 29, 1937 2,156,376 Crosby May 2, 1939 2,397,840 Crosby Apr. 2, 1946 2,397,841 Crosby Apr. 2, 1946 2,416,911 Crosby Mar. 4, 1947 

